1. Field of the Invention
The present invention relates generally to low volatile organic compound (VOC) content, isocyanate based curable oil-in-water emulsions of a polyisocyanate crosslinker and a surface active isocyanate reactive material in an aqueous medium. The present invention also relates to methods for preparing such curable compositions as well as various end uses thereof, and a water-in-oil emulsion precursor suitable for use in preparing such curable compositions.
2. Description of the Related Art
Isocyanate crosslinked systems are in general well known. As an example, polyurethane films can be formed from coating compositions based upon polyols and polyisocyanate crosslinking agents. Polyurethane coatings can be formulated to provide fast curing characteristics, as well as a desirable combination of abrasion resistance, flexibility, adhesion, chemical resistance and appearance characteristics in the resulting films.
Due to the reactivity of isocyanates with active hydrogen containing compounds, including water, polyurethane coatings have historically been formulated as two component organic solvent based systems. One-component systems, both organic solvent based and waterborne (see GB 1530021 and GB 1530022), have also been formulated by blocking of the isocyanate groups via well-known blocking agents.
Despite the excellent films which can be achieved with some of these systems, and particularly the two-component organic solvent based systems, the coatings industry is under great pressure to reduce the environmentally undesirable emission of VOC's, which includes the organic solvent media as well as the common isocyanate blocking agents. One means of doing so, of course, would be to exchange some or all of the liquid organic solvent medium with water. Unfortunately, the switch from organic solvents to water is neither a simple nor straightforward matter, particularly in the case of common isocyanate crosslinkers which are not only reactive with water, but are also hydrophobic and nondispersible.
Several approaches to lowering the VOC of polyurethane coatings are discussed N. T. Cullen, "Low-VOC Polyurethane Coatings: Current Successes and Promising Developments," American Paint & Coatings Journal, Aug. 19, 1991, pp. 44-49 and 64. One such approach has been to preform a water-dispersible film-forming polyurethane polymer by reacting a polyisocyanate with a hydrophilic reactive component, then dispersing the so-preformed hydrophilic polymer in water. A variation on this approach disclosed in GB1162409 is to preform the polyurethane polymer in situ in the aqueous medium with the aid of non-reactive surfactants. A still further variation on this approach is described in EP-A-0369389, in which a lower molecular weight water-dispersible prepolymer containing residual isocyanate functionality is first formed by reaction of a mixture of isocyanates with a polyol chain containing hydrophilic groups, after which the prepolymer is dispersed in water and chain extended or crosslinked. Upon application of these preformed polymer systems to a substrate, films are formed primarily via physical drying mechanisms due to evaporation of the liquid medium (water). While such preformed polyurethane systems can significantly reduce emitted VOC's, they often can suffer from application and stability problems. In addition, films produced from such systems can suffer from poor water resistance due to the hydrophilic nature of the preformed polymers or surfactants remaining after cure.
In another approach disclosed in GB-A2018796 and U.S. Pat. No. 4,663,377, an emulsifiable polyisocyanate mixture comprising (a) a hydrophilic isocyanate functional oligomer and (b) a polyisocyanate, is produced by partially reacting a polyisocyanate with, for example, a hydrophilic polyether alcohol. Curable coating and adhesive compositions can be formed by combining these polyisocyanate emulsions with separate aqueous resins. The emulsifiable polyisocyanate mixtures of these references, however, suffer from low isocyanate content which results from the destruction of some of the isocyanate groups when the polyisocyanate is allowed to partially react with the polyether, as well as the destruction of some of the isocyanate groups due to the reaction with water upon and after emulsification. Predictably, a low isocyanate content would severely reduce the ability of these polyisocyanate mixtures to function as efficient crosslinkers in coating compositions. In addition, these polyisocyanate emulsions suffer from stability problems due to the reaction of isocyanate groups with water, particularly those on the hydrophilic component (a).
In a similar approach disclosed in U.S. Pat. No. 5,202,377, an emulsifiable polyisocyanate mixture comprising (a) a hydrophilic tertiary isocyanate functional oligomer and (b) a polyisocyanate having tertiary isocyanate groups, is produced by partially reacting a polyisocyanate containing tertiary isocyanate groups with a hydrophilic polyether. Coating compositions can be formed by combining (i) these polyisocyanate emulsions with (ii) separate aqueous solutions, emulsions or dispersions of film-forming polymers containing isocyanate-reactive functionality. These emulsifiable mixtures are said to produce more stable emulsions than those of U.S. Pat. No. 4,663,377 due to the lower reactivity of tertiary isocyanate groups. Despite the lower reactivity of the tertiary isocyanate groups, coatings produced from these emulsions may still suffer from low isocyanate content as well as stability problems.
Still another approach to reducing the VOC of isocyanate crosslinked systems is found in U.S. Pat. No. 5,075,370. This reference generically discloses an aqueous coating composition comprising an aqueous solution and/or dispersion of a surface active isocyanate reactive resin (anionic olefinic polyol) into which a specific relatively low viscosity liquid unblocked polyisocyanate crosslinker is emulsified. The disclosed aqueous coating compositions are produced by emulsifying the isocyanate crosslinker into the aqueous solution and/or dispersion of the isocyanate reactive resin to produce an oil-in-water emulsion. It has, however, been found that, when systems were prepared in accordance with the teachings of U.S. Pat. No. 5,075,370 (emulsification of the isocyanate into the aqueous resin solution/dispersion), the isocyanates do not properly incorporate into the resin solution/dispersion nor is an acceptable emulsion produced. After a short period of time a two phase system results, and films obtained from such systems display poor appearance characteristics, are hazy, contain microblisters, and have little or no gloss.
A solution to many of the problems associated with the systems disclosed in U.S. Pat. No. 5,075,370 has been described in U.S. Pat. No. 5,466,745. This patent teaches a curable aqueous oil-in water emulsion prepared by admixing an aqueous medium with a non-aqueous, emulsifiable composition comprising an unblocked polyisocyanate crosslinking agent and a surface active isocyanate reactive material. While excellent stability and ultimate film properties can be achieved with the systems of this patent, viscosity and mixing constraints of the emulsifiable composition may require the addition of diluents prior to admixture with the aqueous medium, thereby undesirably increasing the VOC of the final formulated composition.
All of the aforementioned references are incorporated herein for all purposes as if fully set forth.
It would, therefore, be highly desirable to achieve the excellent stability and film properties of the systems of U.S. Pat. No. 5,466,745, but at overall lower VOC contents of 2.1 lbs/gal (252 g/l ) or less.